The invention relates to s magnetic resonance tomography method where a first slice-selective rf pulse and a second rf pulse act on an examination zone in the presence of a uniform, steady magnetic field, after which further slice-selective pulses are applied to the examination zone, which further pulses excite the nuclear magnetisation in mutually parallel sub-slices which extend perpendicularly with respect to the slice, and where the stimulated echo signals which occur in the associated sub-slice subsequent to each further rf pulse are received and subjected to a Fourier transformation, and also relates to a magnetic resonance tomography apparatus for performing the method.
A method of this kind is known from EP-PS No. 184 810 which corresponds to U.S. Pat. No. 4,748,409. The nuclear magnetisation distribution in an examination zone can thus be measured by means of a single "shot", i.e using only one sequence; therefore, in comparison with the conventional magnetic resonance tomography methods which involve a plurality of sequences the measuring time is short. On the other hand the measuring time is longer than in the method which is known from the magazine "Magnetic resonance in medicine" 5, pages 563 to 571, 1987 where the data must be acquired within a period of no longer than approximately 40 ms. This method necessitates the use of magnetic gradient fields having an extremely large gradient which must be switched within an extremely short period of time when a given spatial resolution is to be obtained. The associated variations in time of the magnetic flux density are a multiple of the guidelines issued by various countries, such as the guidelines product by the FOA (USA). Because the measuring time is substantially longer in the method described in the preamble, the requirements imposed as regards the strength of the gradient field and the rate of change of field strength are substantially less severe
In the known method described above each of the further rf pulses generates not only the desired stimulated echo signal but also a so-called FID signal which relates to the free induction decay, that is to say throughout the entire sub-slice excited by this rf pulse. This undesirable FID signal is slightly dephased by the magnetic gradient field which is generated after each further rf pulse until the beginning of the reception of the stimulated echo signal, referred to hereinafter as the STE signal. Therefore, it is received and subjected to Fourier transform together with the STE signal, so that errors occur in the nuclear magnetisation distribution determined or artefacts appear in the image of this distribution.